News that one of the London suicide bombers studied at a Muslim religious school - or madrassa - in Pakistan has once again raised questions over the country's sprawling system of religious education and what it is producing.

Shehzad Tanweer's family said that he attended a madrassa for two months in the city of Lahore. That has led to intense media speculation in Britain that he may have been "brainwashed" into carrying out the attacks.

It is widely acknowledged that most madrassas are moderate institutions, providing much needed education and board and lodgings for poorer students. The Koran is studied intensively.

But are some of them "breeding-grounds of terror"?

It is estimated that there are now around 20,000 madrassas in Pakistan, compared to around 137 at the time of partition.

According to the Pakistani newspaper, The News, there are today around 1.7m students who attend such institutions, mainly from poor rural families.

Many students develop an intolerant, prejudiced... and narrow-minded view of the world

The reasons for the huge growth in the number of madrassas dates back to 1979, when the Soviet Union's invasion of Afghanistan led to large amounts of money flowing into Pakistan from the West and countries in the Gulf.

Much of this money was directed towards madrassas, and was used by anti-Soviet Mujahideen groups to provide religious and military training for thousands of young fighters prepared to fight the Russians.

Students ('talebs') from Pakistani madrassas were often in the frontlines of the Mujahideen groups that drove the Soviets out of Afghanistan.

Most members of the Taleban government overthrown by the Americans following the 11 September, 2001 attacks in the United States had attended madrassas in Pakistan.

Narrow-minded

Hardliners trained in madrassas have also been blamed for outbreaks of sectarian violence over the last decade in Pakistan in which hundreds of Shias and Sunnis have been killed.

Islamic madrassa schools have multiplied since partition

Critics of the madrassas focus on the narrow curriculum often taught. "Many students develop an intolerant, prejudiced... and narrow-minded view of the world," says Pakistani journalist Ahmed Rashid.

He says that a few hardline madrassas in Pakistan employ teachers sympathetic to al-Qaeda, who encourage students to join extremist groups in Kashmir and Chechnya.

"They gradually become radicalised through this process," he says, "so that it would be no surprise if they ended up joining al-Qaeda."

Many conservative Pakistani families in Britain and elsewhere in the West send their children to madrassas in Pakistan for between six to nine months to complete their children's education.

"The trouble is that many, like Shehzad Tanweer, find this process disorientating and end up more confused than when they arrived," Ahmed Rashid says.

Some madrassas are accused of being Pro-Taleban

In 2001 - shortly after the attack on the World Trade Centre - President Musharraf pledged to reform Pakistan's network of madrassas by making it compulsory for them to register with the authorities and by introducing a wider syllabus which would not focus entirely on studying the Koran.

But the BBC's Haroon Rashid in Peshawar says that there is "little visible sign" that this has been accomplished.

He says most reforms are "cosmetic" due to the reluctance of many madrassas to accept government interference and the desire of the authorities not to alienate influential Muslim organisations.

Two years ago our correspondent visited the Darul Uloom Haqqania seminary in North West Frontier Province, known by students and its principal as the University of Holy War.

Levels of hatred

Here students dedicate their lives to "jihad" and "killing infidels and enemies of Islam".

The European Parliament in April refused to meet a delegation of Pakistani MPs that included Senator Maulana Sami ul-Haq, the head of the "university", because of its links to the Taleban.

Mr ul-Haq was snubbed by the European Parliament

"Although institutions like this are not quite so upfront about their activities today, there is no doubt that levels of hatred and extremism against the West remain," Mr Rashid says.

Experts like Burhanuddin Hasan, former Director of Pakistan Television, stress that the overwhelming majority of Pakistan's madrassas are moderate and provide education to students who otherwise would not have the opportunity.

"But since they are taught neither English nor Science, they risk becoming complete misfits in modern Pakistani society," he argues.

"These days they cannot render any useful service to the nation other then performing religious rites at marriages and burials."